The advent of practical digital telephony was first manifest in trunking of digitally encoded voiceband signals between analog signal switching offices. Subsequently the telephone network has been and continues to evolve toward a primarily digital signals network with new and replacement switching offices being of the digital type. Recently the concept of an integrated services digital network (ISDN) has been widely discussed and published. Practical implementation of ISDN requires a wide band communication path between a subscriber's station set or terminal and the associated switching office. Various means for providing the required wide band transmission path may include the use of optical fiber or coaxial cable. However the fact remains that a near future scrapping of the in place copper twisted pair local distribution telephony network and replacing it with an alternative will be so expensive as to be cost prohibitive. It is self-evident that provision of wide band subscriber lines by such replacement must therefore proceed if at all at a slow and fundable pace. A practical alternative is that of expanding the operational band width of the already in place twisted pair copper conductor subscriber line. Sophisticated termination apparatus applied at the ends of a subscriber line and utilization of the AMI signal format is capable of providing for bit rates in excess of 100 Khz in full duplex four-wire and two-wire configurations. Various publications are directed toward illustrating the required terminal appratus, one such publication being U.S. Pat. No. 4,528,676 entitled "Automatic Correction Circuit for Received Digitally Encoded Signals" issued to Gordon Mein et al on 9 July, 1985.
Telephone subscriber lines for the most part are provided by insulated copper wires being twisted into pairs, each pair being part of a multipair conductor. Sometimes in excess of 100 such pairs are bound togehter in a cable. Interconductor, inductive and capacitive coupling characteristics are avoided to an extent practical, however crosstalk between the pairs, not normally a problem at analog voice frequencies, is one deleterious limitation on the wide band operation of the subscriber line. However the effect of crosstalk, particuarly near-end crosstalk, at the AMI receiver is random and not directly compensatable by methods and apparatus such as automatic line build-out circuits, adaptive equalizers or echo cancellers.